1. Field of the Invention
The present invention relates to a Josephson junction device which can be used in a Josephson computer, a mixer, a sensor and others, and more specifically to a stacked Josephson junction device composed of a non-superconductor layer sandwiched between a pair of superconductor layers each formed of an oxide superconductor material.
2. Description of Related Art
A Josephson junction can exist in various structures. Among the various structures, in practice the most preferable structure is a stacked junction realized by a thin non-superconductor layer sandwiched between a pair of superconductors. In general, the Josephson junction is reduced into practice in the form of a Josephson junction device. A stacked Josephson junction device is comprised of a first superconductor thin layer, a non-superconductor thin layer and a second superconductor thin layer stacked in the named order.
In the above mentioned Josephson junction device, each of the superconductor layers can be formed of any superconductor material, for example, metal type superconductor materials, and so-called compound copper oxide superconductor materials. The non-superconductor layer can be formed of a material selected from a group consisting of insulator materials, semiconductor materials and normal conductor materials, depending upon use of a superconducting device.
Lately, in the stacked Josephson junction devices, various attempts have been made which use a copper oxide superconductor material for the superconductor layers and an oxide typified by MgO and ZrO.sub.2 for the non-superconductor layer.
In the stacked Josephson junction the thickness of the non-superconductor layer is determined by a property of the non-superconductor layer itself and a coherence length of the superconductor. For example, if the coherence length of the superconductor is short, the thickness of the non-superconductor layer must be made small. In general, copper oxide superconductor materials have a very short coherence length (several angstroms in a c-axis direction and several tens of angstroms in a direction perpendicular to the c-axis). Therefore, if the non-superconductor layer is formed of an insulator material, the thickness of the non-superconductor material layer must be very thin, about 50 .ANG. at maximum. Even in the case that the non-superconductor layer is formed of a normal conductor material, the thickness of the non-superconductor material layer must be very thin.
In the conventional stacker Josephson junction having each superconductor layer formed of the copper oxide superconductor material, however, the non-superconductor layer is not uniform, and therefore, it has been difficult to obtain an excellent junction. The reason for this is considered to be that when a thin layer of the oxide such as MgO or ZrO.sub.2 used for forming the non-superconductor layer is deposited on the copper oxide superconductor, the oxide grows in the form of islands.
The non-superconductor layer grown the form of islands has pin holes and different in thickness from one location to another, which results in a significant adverse effect to the junction.